The effects of methylmalonate (MMA) on succinate dehydrogenase (SDH) and beta-hydroxybutyrate dehydrogenase (HBDH) activities in brain and liver of 15-day-old rats were studied. The apparent Km of SDH for succinate was 0.45 mmol/L in brain and 0.34 mmol/L in liver. MMA inhibited the enzyme activity in both tissues with Ki values of 4.5 mmol/L and 2.3 mmol/L in brain and liver, respectively, and the inhibition was of the reversible competitive type. The calculated Km for HBDH with beta-hydroxybutyrate as substrate was 1.26 mmol/L in brain and 0.36 mmol/L in liver. MMA inhibited the enzyme with a Ki value of 0.015 mmol/L in brain and 0.275 mmol/L in liver. These results are probably relevant to our understanding of cerebral metabolism in methylmalonic acidaemic children, especially during ketoacidotic and hypoglycaemic crises, and may be related to the pathogenesis of cerebral dysfunction of methylmalonic acidaemia.

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Methylmalonate (MMA) accumulates in the tissues of patients with methylmalonic acidaemia, who present severe neurological signs soon after birth and later mental retardation. Attempting to understand the pathophysiology of the disorder, we investigated the effects of MMA on brain glucose uptake, lactate release and CO2 production. Glucose uptake and lactate release were studied by incubating 40 microns wide brain prisms from 15-day-old rats in Krebs-Ringer bicarbonate buffer, pH 7.0, containing 5.0 mmol/L glucose and one of three concentrations of MMA (1.0, 2.5 and 5.0 mmol/L). Controls did not contain MMA in the incubation medium. MMA induced a significant increase of lactate production in a dose-dependent pattern that was proportional to glucose uptake by the brain prisms. We also studied the influence of MMA on brain CO2 production from [2-14C]glucose and [U-14C]acetate by incubating brain prisms in the same buffer in the presence of the substrates with (experimental groups) or without (controls) 5.0 mmol/L MMA. MMA significantly reduced CO2 formation from both substrates.

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Methylmalonic acidaemia is an inherited metabolic disorder generally associated with severe clinical features. Those who survive the first months present a variable degree of mental retardation (Rosemberg, 1983). Methylmalonate (MMA) is the major metabolite which accumulates in tissues of methylmalonic acidaemic patients. To our knowledge, only two studies have been done on the effects of MMA on brain metabolism (Frenkel et a/., 1973; Patel et a/., 1976). Since glucose is the major substrate that throughout life supports the energy requirements of the brain, we studied the effects of MMA on glucose uptake in rat brain in vitro.,Wistar rats bred in our laboratory were used in this study. Free water and a 20% (w/w) protein commercial chow were provided. The mother and eight pups were kept per cage. Half of the animals were left fasting 24 h before the experiments. The rats were killed by decapitation without anaesthesia at 15 days of age, their brains were rapidly removed. The olfactory bulbs, cerebellum and pons/medulla were discarded. The rest of the brain (cerebrum) was cut in two perpendicular directions with a Mcllwain chopper into prisms of 40 p m width. Brain prisms from fed and fasting animals were incubated under 0 , / C 0 2 (19:l) mixture at 37°C for 30 min in Krebs-Ringer bicarbonate buffer, pH 7.0 (100 mg of tissue/ml of buffer) containing: glucose 5.0 mM and one of the three concentrations of MMA, 1.0, 2.5 and 5.0 mM. Control experiments did not contain MMA in the incubation medium. Glucose was measured by the glucose oxidase method (Trinder, 1969) and the uptake determined by subtracting the amount found after the incubation from the total amount placed in the medium.It can be seen in Table 1 that MMA had no effect on glucose incorporation by brain prisms of fed rats, but, surprisingly, it increased glucose uptake by brain prisms of fast-Abbreviation used: MMA, methylmalonate.ing rats at levels as low as 1.0 mM. Further assays revealed that MMA doses of 0.5 mM also produced such an effect. On the other hand, propionate in doses up to 10.0 mM did not affect glucose incorporation (results not shown), ruling out a possible non-specific effect due to an acidic component added to the incubation medium. Both MMA and propionate in the doses used did not change the buffer pH. It can also be seen in the Table that the brains of fasting rats incorporated less glucose in the control experiments (incubation medium free from MMA).In another experiment, we applied three subcutaneous injections of buffered MMA, pH 7.2, with an interval of 90 min between them, to 15-day-old fed and 24 h fasting rats in such doses (10.5 mg of MMA/100 g body weight) that the animals achieved MMA serum levels of around 2.5 mM (concentration found in methylmalonic acidaemic patients at crises). Control animals received saline in the same volumes. Ninety minutes after the last injection animals were decapitated and the brain processed as before, excepting for the presence of MMA in the incubation medium. Again we observed that only brain...